The condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products is a widely used commercial process. This type of condensation reaction is referred to herein as a polymerization reaction, and the products can be either low molecular weight oligomers or high molecular weight polymers. Oligomers are formed by the condensation of 2, 3 or 4 olefin molecules with each other, while polymers are formed by the condensation of 5 or more olefin molecules with each other. As used herein, the term "polymerization" is used to refer to a process for the formation of oligomers and/or polymers. Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butene) can be converted by polymerization over a solid phosphoric acid catalyst to a product which is comprised of oligomers and is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products which include alcohols, detergents and plastics.
The acid catalyzed alkylation of aromatic compounds with olefins is a well-known reaction which is also of commercial importance. For example, ethylbenzene, cumene and detergent alkylate are produced by the alkylation of benzene with ethylene, propene and C.sub.10 to C.sub.18 olefins, respectively. Sulfuric acid, HF, phosphoric acid, aluminum chloride, and boron fluoride are conventional catalysts for this reaction. In addition, solid acids which have comparable acid strength can also be utilized to catalyze this process, and such materials include amorphous and crystalline aluminosilicates, clays, ion-exchange resins, mixed oxides and supported acids such as solid phosphoric acid catalysts.
Solid phosphoric acid catalysts are typically prepared by combining a phosphoric acid with a support and drying the resulting material. A commonly used catalyst is prepared by mixing kieselguhr with phosphoric acid, extruding the resulting paste, and calcining the extruded material. The activity of a solid phosphoric acid catalyst is related to the amount and the chemical composition of the phosphoric acid which is deposited on the support. Phosphoric acid consists of a family of acids which exist in equilibrium with each other and differ from each other in their degree of condensation. These acids include ortho-phosphoric acid (H.sub.3 PO.sub.4), pyro-phosphoric acid (H.sub.4 P.sub.2 O.sub.7), triphosphoric acid (H.sub.5 P.sub.3 O.sub.10), and polyphosphoric acids, and the precise composition of a given sample of phosphoric acid will be a function of the P.sub.2 O.sub.5 and water content of the sample. As the water content of the acid decreases, the degree of condensation of the acid increases. Each of the various phosphoric acids has a unique acid strength, and, accordingly, the catalytic activity of a given sample of solid phosphoric acid catalyst will depend on the P.sub.2 O.sub.5 /H.sub.2 O ratio of the phosphoric acid which is deposited on the surface of the catalyst.
The activity of a solid phosphoric acid catalyst and also its rate of deactivation in a hydrocarbon conversion process, such as a polymerization or an alkylation process, will be a function of the degree of catalyst hydration. In an olefin polymerization process, a properly hydrated solid phosphoric acid catalyst can be used to convert over 95% of the olefins in a feedstock to higher molecular weight products. However, if the catalyst contains too little water, it tends to have a very high acidity, which can lead to rapid deactivation as a consequence of coking, and the catalyst will not possess a good physical integrity. Further hydration of the catalyst serves to reduce its acidity and reduces its tendency toward rapid deactivation through coke formation. However, excessive hydration of a solid phosphoric acid catalyst can cause the catalyst to soften and physically agglomerate and, as a consequence, can create high pressure drops in fixed bed reactors. Accordingly, there is an optimum level of hydration for a solid phosphoric acid catalyst.
During use as a catalyst for a hydrocarbon conversion process, a solid phosphoric acid catalyst will develop a degree of hydration which is a function of feedstock composition and reaction conditions. For example, the level of hydration is affected by the water content of the feedstock which is being contacted with the catalyst and also by the temperature and pressure at which the catalyst is used. The vapor pressure of water over a solid phosphoric acid catalyst varies with temperature, and it is important to keep the water content of the hydrocarbon process stream in equilibrium with that of the catalyst it is being contacted with. If a substantially anhydrous hydrocarbon feedstock is used with a properly hydrated catalyst, the catalyst will typically loose water during use and will develop a less than optimal degree of hydration. Accordingly, when the water content of a feedstock is inadequate to maintain an optimal level of catalyst hydration, it has been conventional to inject additional water into the feedstock. A study of the effect of water on the performance of solid phosphoric acid catalyst as a catalyst for the alkylation of benzene with propene and for the oligomerization of propene is set forth in a review article by Cavani et al., Applied Catalysis A: General, 97, pp. 177-196 (1993).
As an alternative to incorporating water into a feedstock that is being contacted with a solid phosphoric acid catalyst, it is also conventional practice to add a small amount of an alcohol, such as 2-propanol, to the feedstock to maintain the catalyst at a satisfactory level of hydration. For example, U.S. Pat. No. 4,334,118 (Manning) discloses that in the polymerization of C.sub.3 -C.sub.12 olefins over a solid phosphoric acid catalyst which has a siliceous support, the catalyst activity can be maintained at a desirable level by including a minor amount of an alkanol in the olefin feedstock. It is also disclosed that the alcohol undergoes dehydration upon contact with the catalyst, and that the resulting water then acts to maintain the catalyst hydration.